A lecture for an Exploit Development class More info: https://samsclass.info/127/127_S21.shtml

1. CNIT 127: Exploit Development
Ch 8: Windows Overflows
Part 1
Updated
4-18-2021

2. Topics
• Thread Environment Block and Process
Environment Block
• Stack-Based Buffer Overflows
• Defeating ASLR (Address Space Layout
Randomization)
• Frame-Based Exception Handlers
• SEH Protections
• Defenses in Server 2008 (not in textbook)

3. Thread Environment Block and
Process Environment Block

4. Thread Environment Block (TEB)
• Stores information about the currently
executing thread
• Also called the Thread Information Block
• Officially undocumented for Win 9x
• Partially documented for Win NT
• So many Win32 programs use
undocumented TEB fields that they are
effectively part of the API

5. Thread Environment Block (TEB)
• TEB can be used to get a lot of
information about a process without
calling the Win32 API
• Can get
• Last error
• Import Address Table
• Process startup arguments
• and more

6. Thread Environment Block (TEB)
• SEH pointer
• Info about the stack
– Much more (Link Ch 8a)

7. FS: and GS:
• Segment Registers
• Left over from very early operating
systems
• Not used much anymore for their original
purpose
• On Windows 32-bit x86, FS: is used to
point to the TEB
– Links Ch 8e, 8y

8. Process Environment Block (PEB)
• An opaque data structure
• Details are hidden from its users
• Values are only intended to be
manipulated by calling subroutines that
can access the hidden information
• Used by Win NT internally
• Most fields intended only for internal OS
use

9. Process Environment Block (PEB)
• Only a few fields are documented by
Microsoft
• Contains data structures that apply across
a whole process

10. Process Environment Block (PEB)

11. Stack-Based Buffer Overflows

12. Classic Technique
• Overwrite the saved return address
– With a value that points back into the stack
• When the function returns, it copies the
return address into EIP
• Return address points to NOP Sled

14. SEH Technique
• Overwrite the SEH
– With a value that points back into the stack
• Trigger an exception
• Modified exception handler points to NOP
Sled

16. Defeating ASLR (Address
Space Layout Randomization)

18. Using Return Value
• We're inside the
function
• ESP points to the
stack
• Find a JMP ESP and
insert its address
into the return
value

19. Using SEH
• Old Windows versions
left the address of
the SEH in EBX
• Newer versions clear
all registers
• But SEH address is at
ESP+8
• Find POP, POP, RET
and put its address in
SEH

20. Frame-Based Exception
Handlers

21. Exception Handler
• Code that deals with problems from
within a running process
– Such as access violation or divide by zero
• Frame-Based Exception Handler
– Associated with a particular procedure
– Each procedure sets up a new stack frame
• Every thread in a Win32 process has at
least one frame-based exception handler

22. EXCEPTION_REGISTRATION
• Each thread's TEB has the
address of the first
EXCEPTION_REGISTRATION
structure at fs:[0]
• When an exception occurs,
the OS walks through the
list until a suitable handler
is found

23. SEH
Example
Code

24. SEH Chain in Immunity
• View, SEH Chain
• This is Notepad's SEH Chain

25. Follow Address in Stack

26. Exceptions in Stack Overflows
• Suppose we overflow the stack to
overwrite the stored return address
• Unfortunately, we also overwrite other
variables by accident (collateral damage)
• Those may lead to exceptions, as other
instructions become invalid, before the
function returns

27. Overwriting EXCEPTION_REGISTRATION
• To gain control of exception handling
• On Windows 2000 and Win XP (before SP1)
– EBX points to the current
EXCEPTION_REGISTRATION structure
• On later Windows versions, all the
registers are zeroed when the SEH is
called
– In order to make exploitation more difficult

28. Old Windows Process
• At crash, EBX points to
EXCEPTION_REGISTRATION structure
• Overwrite
With "JMP EBX"

29. Modern Windows Process
• Stack at crash
• 3rd value points
to EXCEPTION
_REGISTRATION
structure
• Use POP, POP, RET

30. SEH Protections

31. Win 2003 Server
• Attempts to ensure that handler is valid
before using it, with these steps
1.If handler is on the stack -- INVALID
– According to the TEB's entries FS:[4] and FS:{8]
2.If handler is in any loaded EXE or DLL –
MAYBE
– Otherwise VALID

32. Win 2003 Server
3. If the module is marked as "not allowed –
INVALID
4. If a module has no "Load Configuration
Directory", or one with a small size: VALID

33. Three Ways to Exploit SEH on Windows
Server 2003
1. Abuse an existing handler
2. Use code in an address outside all
modules
3. Use code in a module without a "Load
Configuration Directory"

34. Defenses Added in Win Server
2008
Not in Book

35. Microsoft's Defenses
• Normal SEH attack (link Ch 8f)

36. SAFESEH
• Microsoft added this compiler switch to
Visual Studio 2003
• It creates a whitelist of exception handler
addresses
• BUT this depends on the developer using
the switch
– All legacy code must be recompiled

37. SEHOP
• Verifies that the exception handler is
intact before using it
• The pointer to the next handler comes
before the pointer to the registration
record
• So an exploit will usually damage the
*Next pointer

38. EXCEPTION_REGISTRATION_RECORD

39. How SEHOP Works
• Adds an extra registration record at the
end of the list
• Walks the exception handler list to ensure
that the added record can be reached
• Will detect corruption of *Next pointers

40. SEHOP

41. Windows Versions
• SEHOP is enabled by default in Windows
Server 2008
• Disabled by default in Windows Vista
– Because it breaks some legacy code
• Also disabled by default on Windows 7 and
10
– But it can be enabled in the Registry
– Link Ch 8k